Reaction to the results of the Programme for International Student Assessment (Pisa) has been, predictably, varied. On the one hand, it has been argued the decline in performance of our 15-year-old pupils appears to have been halted and we are slightly above the Organisation for Economic Co-operation and Development (OECD) average in reading and science and average in mathematics. On the other, it has been suggested that Scottish education, after years of investment and change, is simply “treading water”. However, a closer analysis of the data from the tests carried out in 2009 throws up some fundamental challenges for Scottish schools.

The executive summary of the Pisa results produced by the OECD has a section headed “What makes a school successful? Resources, policies and practices”. The first paragraph argues that comprehensive systems of schooling which “embrace diverse student populations” tend to “show high performance and an equitable distribution of learning outcomes”. Specifically, the report argues that in systems where pupils have to “repeat grades”, results are worse and “socio-economic differences in performance tend to be wider”.

Perhaps the most telling finding of all for Scottish schools, however, is that “where 15-year-olds are divided into more tracks based on their abilities, overall performance is not enhanced, and the younger the age at which selection for such tracks first occurs, the greater the differences in student performance, by socio-economic background, by age 15, without improved overall performance”.

Finally, in school systems where it is more common to “transfer weak or disruptive students out of a school, performance and equity tend to be lower”.

Thus, while it would appear that we can take some comfort from the fact that our system of comprehensive education remains the best way of tackling underachievement and social exclusion, we are reminded that internal selection - or setting - remains problematic. We already know that there is no convincing research evidence to support the practice of setting by prior attainment, and yet it continues to be the default position in our schools. Now, the findings from Pisa 2009 remind us that not only is there no evidence that it improves attainment, but it makes the system less equal by increasing the differences of pupils of different socio-economic status.

A worrying statistic in the report is that 16 per cent of 15-year-olds in Scotland are performing below Pisa level 2 in reading, the level which they argue is necessary for individuals to be able to play a full part in modern society. While this is marginally less than the percentage - 19 per cent - across all countries, it still indicates that the lowest-achieving pupils, often placed in lowest sets, are not being well served by the system.

In their 2009 book, The Spirit Level, Richard Wilkinson and Kate Pickett argue that “international education scores are closely related to income inequality”. In other words, it is not whether the country itself is richer or poorer than other countries, it is about how equal or unequal the country itself is. The authors use the 2000 Pisa study to demonstrate the relationship between overall performance and inequality in the countries. They conclude that “poverty does not explain the inequality effect”.

They refer to the work of Doug Wilms, professor of education in New Brunswick (and a significant contributor to the school effectiveness debate in Scotland) and suggest that “an important influence on average literacy scores - on national levels of achievement - is the steepness of the social gradient”. Wilkinson and Pickett look also at what they call “unequal learning opportunities”, including the stereotyping of pupils, and conclude that “inequalities . in society and in our schools, have a direct and demonstrable effect on our brains, on our learning and educational achievement.”

On the positive side is the data which indicates a number of countries in the sample that have narrowed the gap in performance between high and low socio-economic groups. Less so is the data which indicates the continuing gap in achievement in reading between girls and boys.

So, what is to be done? The report offers no simple solutions but, surely, one message is clear. Setting by prior attainment does nothing to raise attainment or narrow the gap. Bottom sets are disproportionately populated by boys from the lowest socio-economic groups, the very groups which are under-represented in our universities and over-represented in our exclusion statistics and in our prisons.

As the subtitle of The Spirit Level puts it, “Equality is Better for Everyone”. Institutionalised inequality has no place in a modern, inclusive education system.

This may be as much an issue of political will as professional practice. If comprehensive schools do best for all pupils and internal selection, making pupils repeat grades and competition among schools do not produce better results, then we should abolish practices which reinforce inequality.

The messages from Pisa 2009 are challenging, but solutions are achievable. First, reading is the key to educational success; encouraging reading in early years and sustaining it thereafter is crucial. Second, Curriculum for Excellence, initiated by Labour and commanding cross-party support in principle, appears to be on the right lines. The emphasis on depth, on higher-order skills and on raising aspirations is fundamental. Finally, the link between poverty and underachievement can be broken if schools have an inclusive climate, promote positive behaviour and have high aspirations for all pupils.

Brian Boyd is emeritus professor of education at Strathclyde University.